Electrical contact brush



waxy-"a oe United States Patent ELECTRICAL CONTACT BRUSH Maurice R. McCrary, Bement, 111., and Dimiter Ramadanolf, Berea, Ohio, assignors to Union Carbide Corporation, a corporation of New York No Drawing. Application August 1, 1952, Serial No. 302,246

Claims. (Cl. 117-228) This invention relates to an electrical contact brush of the type generally used on motors and generators. More particularly, the invention relates to an electrical contact brush which has exhibited excellent operating properties at high altitudes as well as at sea level.

The performance of conventional electrical contact brushes made of carbon or graphite is known to be affected to a considerable extent by atmospheric conditions. A brush which is in every way satisfactory at sea level under normal conditions will often prove to be inoperable at altitudes of sevenal thousand feet above sea level. Many brushes disintegrate, or dust as it is descriptively referred to, at altitude easily obtainable with todays aircraft. This behavior is believed to be attributable to the paucity of oxygen and water vapor at these altitude since oxygen and water contribute to the formation of a film on the surface of a commutator. A film on which the brush rides is essential to proper commutation and brush performance.

The prior art teaches the addition of certain materials to carbon or graphite brushes to improve their operating properties. Metals such as silver and copper have been added to change the resistance of the brush and to lower the contact drop between the brush and the commutator. Other additives have been used to alter or form a film on the surface of a commutator. So e treatments which are excellent for brushes to be use at high altitudes render them unsatisfactory for operating at sea level. The reverse is also true.

The principal object of the present invention is to provide an electrical contact brush with generally improved operating properties. Another and more specific object of the invention is to provide an electrical contact brush which gives excellent service at high altitudes.

The objects of the invention are accomplished by including in a conventional carbon or graphite brush a compound of an alkaline earth metal and a compound of silver.

The preferred embodiment of the invention is an electrographitic brush containing calcium carbonate and silver sulfide. Best results have been obtained with an electrographitic brush containing about six percent by weight of calcium carbonate and about three and one-half percent silver sulfide. This brush has exhibited excellent commutating properties at sixty thousand feet above sea level as well as at sea level.

The following treatment is an example of the preferred manner in which brushes of the invention may be prepared: Electrographitic brush stock, suitably in the shape and size of the finished brushes, is impregnated with an aqueous solution of silver nitrate. While still wet, the brush stock is soaked in an aqueous solution of ammonium hydrosulfide to convert the silver nitrate to silver sulfide. The brush stock is then dried at a temperature of 150 C. and fired at 650 C. for about three minutes. The amount of silver sulfide remaining in the brush will, of course, depend upon the concentration of the silver nitrate solution employed. For example, a deposit of silver sulfide equaling about 3.5% by weight of the brush stock is obtained with a silver nitrate solution made up of about forty grams of silver nitrate to one hundred cubic centimeters of water. Higher or lower concentrations of silver sulfide in the brush may be obtained by using, respectively, more or less concentrated solutions of silver nitrate.

The silver sulfide impregnated brush stock is then im pregnated with calcium carbonate. This is accomplished satisfactorily by impregnating the treated brush stock with an aqueous solution of calcium nitrate; drying the brush stock at a temperature of 150 C.; and then heating it to reduce the calcium nitrate to calcium carbonate, this heating suitably being to about 650 C. A carbon brush treated with a solution of calcium nitrate having a concentration of about 400 grams of calcium nitrate tetrahydrate to cubic centimeters of water exhibited a calcium carbonate content of about 4% when dried and heated as described above. Because of the high solubility of calcium nitrate in water, it has been found that up to 8% calcium carbonate can be introduced into brush stock with a single impregnation and heating. Aqueous solutions of other salts, such as calcium formate and calcium acetate which decompose on heating to yield calcium carbonate, can be employed to impregnate the brushes of the invention. When high concentnations of calcium carbonate are desired or when solutions containing a small percentage of the soluble salt are used, multiple impregnations may be necessary to introduce sufiicient calcium carbonate into the brush.

The preferred processes described above for producing the brush of the invention are merely illustrative of those that can be employed. The materials need not be introduced into the brush in any particular order. For example, the compound of the alkaline earth metal may be put into the brush before the silver compound. The art is well versed in techniques for impregnating brush stock, and any of the conventional processes may be used in making the brush of the invention.

In addition to silver sulfide, silver chloride in combination with a compound of an alkaline earth metal has given good results under both sea level and high altitude conditions. Other silver salts that are relatively insoluble in water and which do not decompose at operating temperatures can be employed in the brush of the invention. Besides calcium carbonate, other salts of alkaline earth metals in general have proved satisfactory. Strontium carbonate and barium fluoride are examples of salts of alkaline earth metals that have proved to be operable. In the following table examples are given of the various impregnants used and the proportions in which they have been combined in the brush of the invention.

TABLE I Impregnants 10.76% Bar,

EARCH soonobtained with between 0.5 and of the silver compound and between 2% and 10% of the compound of an alkaline earth metal.

It has been found that when barium fluoride is the alkaline earth metal compound in the brush of the invention, best results are not obtained unless the barium fluoride is in someway bonded to the brush stock. A convenient way of effecting such a bond is as follows: after the brush has been impregnated with barium fluoride, impregnate it with a solution of a resin, suitably a phenolic resin, and a volatile solvent; heat the brush to drive off the solvent and coke the resin. The small amount of resin coke thus formed effectively bonds the barium fluoride to the brush stock and insures satisfactory operation of the brush of the invention. It will be understood that many resins can be used, the choice depending on the ease with which they can be coked. Both thermosetting and thermoplastic resins have produced the desired results.

The brush stock which is impregnated to make the brush of the invention can be any of the conventional carbonaceous materials conventionally employed in the making of brushes. Carbon flour, lamp-black, and graphite, both natural and artificial, are examples of such carbonaceous materials. In the appended claims the term carbonaceous brush stock includes all of the conventional brush-forming materials.

What is claimed is:

1. An electrical contact brush comprising We" brush stock impregnated with at least one a a ne earth metal compound selected from the group consisting of calcium carbonate and barium fluoride, and at least one silver-containing compound selected from the group consisting of silver sulfide and silver chloride, said alkaline earth metal compound and said silver-containing compound being present in amounts sufficient to impart high current load capacity and low brush friction properties at high altitudes to said brush. 1

2. An electrical contact brush as described in claim 1 wherein the impregnants are calcium carbonate and silver 4 3. An electrical contact brush as described in claim 1 wherein the impregnants are barium fluoride and silver sulfide.

4. An electrical contact brush as described in claim 1 wherein the impregnants are calcium carbonate and silver chloride.

5. An electrical contact brush as described in claim 1 wherein the impregnants are barium fluoride and silver chloride.

6. An electrical contact brush as described in claim 1 wherein the alkaline earth metal compound is present in an amount equivalent to from 2% to 10% by weight of brush stock and the silver-containing compound is present in an amount equivalent to from 0.5% to 10% by weight of brush stock.

7. An electrical contact brush as described in claim 6 wherein the impregnants are calcium carbonate and silver sulfide.

8. An electrical contact brush as described in claim 6 wherein the impregnants are barium fluoride and silver sulfide.

9. An electrical contact brush as described in claim 6 wherein the impregnants are calcium carbonate and silver chloride.

10. An electrical contact brush "as described in claim 6 wherein the impregnants are barium fluoride and silver chloride.

References Cited in the file of this patent 

1. AN ELECTRICAL CONTACT BRUSH COMPRISING CARBONACEOUS BRUSH STOCK IMPREGNATED WITH AT LEAST ONE ALKALINE EARTH METAL COMPOUND SELECTED FROM THE GROUP CONSISTING OF CALCIUM CARBONATTE AND BARIUM FLUORIDE, AND AT LEAST ONE SILVER-CONTAINING COMPOUND SELECTED FROM THE GROUP CONSISTING OF SILVER SULFIDE AND SILVER CHLORIDE, SAID ALKALINE EARTH METAL COMPOUND AND SAID SILVER-CONTAINING COMPOUND BEING PRESENT IN AMOUNTS SUFFICIENT TO IMPART HIGH CURRENT LOAD CAPACITY AND LOW BRUSH FRICTION PROPERITIES AT HIGH ALTITUDES TO SAID BRUSH. 